This invention relates particularly, but not necessarily only, to carousel-type drill rod racks for rotary drills such as water well drills and blast hole drills. Such racks are well known in the art -- see for example U.S. Pat. No. 2,972,388, issued Feb. 21, 1961 -- and generally comprise upper and lower arms that are pivotably mounted and swingable into and out of working position, a vertical center shaft rotatably supported between the outer ends of the arms, upper and lower support plates on the center shaft that receive the upper and lower ends of the drill rods, and means to index the support plates. When a drill rod is to be added to the string, the rack is swung into position to put a rod into alignment with the rotary head, the head is used to make the threaded connection, and the rack is then swung away so that drilling can proceed. For removal, the drill rod section is raised, the rack is swung into position to receive it, the threaded connection is broken, and the rack is then moved back to remove the drill rod.
Such racks are generally quite satisfactory insofar as they provide a convenient and compact means of storing and handling a plurality of drill rods. Known arrangements, however, have a number of disadvantages. For one thing, known arrangements generally include relatively complex latching mechanisms for holding and releasing the upper ends of the drill rods and these are subject to malfunction. Further, known arrangements do not have fully satisfactory means for breaking the threaded connection between the drive head and the top drill rod section without relying only on the torque of the drive head; in some cases, the arrangements require the provision of flats in addition to those usually found at the top of a drill rod. Still further, it is often difficult for the operator, who is standing near the bottom of the mast, to see whether the upper end of the drill rod is properly connected, disconnected, or otherwise oriented with respect to the upper support plate. In addition, such racks are often used with masts that can be titled to a horizontal position for travel, and in this position it is difficult to prevent forward axial movement of the drill rods, for example when the carrier vehicle stops suddenly.